(1) Field of the Invention
The present invention relates to gas discharge plasma processing, and particularly to plasma etching processes and apparatus.
(2) Description of the Prior Art
Plasma processing has recently become the subject of extensive investigations as a gas phase etching process, because it is superior to wet chemical processes for etching microscopic features, when used in conjunction with a suitable etch mask, in the manufacture of silicon integrated circuits. Present day very large scale integrated circuits (VLSI circuits), such as are used for semiconductor memories and processors, require a manufacturing capability to etch patterns having micron and even sub-micron dimensions.
The typical pattern etching procedure involves first applying a film of a photosensitive, X-ray sensitive, or electron-beam sensitive polymer (called a photoresist, X-ray resist, or electron-beam resist, according to the type of sensitivity) on the surface of a previously deposited layer which is to be etched. This polymer film is then selectively exposed to sensitizing radiation through a selectively opaque pattern or by modulated beam scanning.
Subsequent development of the exposed portions of the resist causes either the exposed or the unexposed portions to be removed, depending on whether the polymer is a positive resist or a negative resist. In either case, the resulting etch mask permits selective etching away of the portions of the underlying layer from which the resist was removed during development. This layer is usually a metal or a dielectric which serves some electrical function in the integrated circuit.
When etching is completed, the remaining resist material is removed by a resist stripping process, leaving behind the unetched portions of the underlying layer in the desired pattern. An integrated circuit is produced by repeated sequences of layer deposition, resist application, exposure, development, etching, and resist stripping.
Plasma methods have several important advantages over wet chemical processes in carrying out resist development (called plasma development), layer etching (called plasma or reactive ion etching), and resist removal (called plasma stripping or plasma ashing). These methods are dry, the removed products are in gaseous form, eliminating clean up and simplifying disposal, and they can be accurately controlled by the level and timing of applying the activating voltage.
One of the most important advantages of plasma processing, however, lies in its anisotropic etching capability. Wet chemical etching is essentially an isotropic process. That is, etching proceeds in all directions at approximately the same rate, laterally as well as perpendicularly to the surface of the layer being etched. This means that undercutting of the etch mask occurs, so that pattern line width resolution less than a few times the thickness of the etch layer is not possible. Although reactive ion etching is also predominantly a chemical process, the etch rate normal to the surface can be much higher than the lateral etch rate because activating agents responsible for the process are delivered to the substrate predominantly in the former direction, for reasons explained below.
Basic to all of the above-described plasma processes is the creation of an electrical gas discharge (plasma) by imposing a direct current (dc) voltage or, preferably, a radio frequency (rf) voltage between electrodes in a space occupied by a normally non-reactive gas at low pressure. Energetic electrons emitted from the negative electrode (i.e., the cathode) collide with neutral gas atoms or molecules to create ions or other reactive species and additional electrons, thereby initiating and maintaining a highly conductive glow discharge in a region adjacent to the cathode. This glow discharge or plasma is separated from the cathode surface by a dark space or plasma sheath.
Since the plasma is essentially equipotential, the voltage drop between the plasma and the cathode occurs in the plasma sheath, and the direction of the electric field is normal to the cathode surface. Consequently, the ions and other reactive species generated in the plasma, which typically carry a positive charge, are attracted to the cathode surface and travel from the plasma to the surface primarily in a direction parallel to the electric field lines. In the plasma processes considered here, the cathode serves as a substrate support, so when the ions or reactive species reach the surface of the substrate they either activate or take part in chemical reactions resulting in the respective resist development, layer etching, and resist stripping.
As an example, plasma stripping is conducted with oxygen as the reactive gas. The electron-molecule collisions in the plasma convert molecular oxygen to atomic oxygen, as well as positive and negative oxygen ions. These reactive forms of oxygen aggressively attack the polymeric resist film, creating gaseous oxidation products such as CO, CO.sub.2, and H.sub.2 O. The resist material is thereby effectively removed from the surface. Similar mechanisms are involved in the developing and etching procedures.
The major drawback to these plasma processes is that they are slow. The efficiency of conversion of inert gas molecules to reactive species is directly proportional to the plasma density, that is, the density of electrons in the glow discharge. In a steady-state plasma, generation of positively charged species and free electrons is balanced by their recombination into neutral gas atoms and molecules plus their loss by transport from the plasma. In a typical diode generated gas discharge only about one in 10.sup.6 of the gas molecules is dissociated into reactive species of the type required for plasma processing. Since the reactive species generation rate is constant in a given plasma, depletion of these species by the chemical reactions taking place in plasma processing causes the net rate to decrease. This is known as the "loading effect," in that the rate of reactive species generation is inverse to the load of material to be reacted.
In addition to its adverse influence on the plasma processing rate, loading effect can also cause serious mask undercutting in the plasma etching process. This condition occurs as normal etching nears completion. At that point, the material of the layer to be etched has been consumed, causing the depletion rate of the reactive species to drop suddenly when the underlying inert substrate material is exposed. The loading effect produces a corresponding rapid increase in reactive species availability, resulting in an abrupt increase in the lateral etch rate.
Loading effect can be reduced by increasing the reactive species generation rate. One way to do this is by increasing the pressure of the reactive gas, thereby increasing the density of molecules available for reactive species generation. Increasing the pressure reduces etch anisotropy, however, because the reactive species or activating ions have an increased probability of colliding with a gas molecule on their way through the plasma sheath, so that they impact the etch layer surface at an oblique angle.
The rate of reactive species generation also can be increased, and the loading effect concomitantly reduced, by increasing the plasma density. It is well known to increase plasma density in cathode sputtering processes by the use of a magnetic field. This causes a spiraling electron path and thus increases the probability of an ionizing collision with a gas molecule or atom. Particularly effective for increasing the ionization efficiency of plasmas are electron-trapping magnetic fields in which the lines of magnetic force cooperate with the cathode surfaces to form a completely enclosed region, preferably in which the magnetic field is orthogonal to the electric field.
These conditions are met in cylindrical post-type and hollow cathode-type magnetron sputter devices, as first disclosed by Penning (U.S. Pat. No. 2,146,025 issued on Feb. 7, 1939). Variations of the Penning structure and a more complete discussion of the theory involved are given in U.S. Pat. No. 4,041,353 issued to Penfold and Thornton on Aug. 9, 1977. The design of magnetic trapping fields for planar sputter targets is disclosed in U.S. Pat. No. 3,878,085 of Corbani and U.S. Pat. No. 4,166,018 of Chapin.
Sputtering is predominantly a physical process, however. It has been likened to sand-blasting on the molecular scale because it depends on the kinetic energy of positive ions, attracted to and striking a highly negative cathode, to dislodge neutral target atoms which then ultimately condense on the surface of a substrate exposed to the cathode.
The kinetic energy required for the chemical reactions involved in plasma processing are much lower, however, than the energies typically encountered in diode sputtering (several electron volts as compared with several hundred ev). The excess ion energy available in a sputtering system, therefore, would merely generate heat if used for plasma etching. This is highly undesirable because the polymeric materials used for etch masks cannot generally be used at temperatures above about 125.degree. C.